A2-Level Transition Elements

  • In homogeneous catalysis, a catalyst is in the same phase as the reactants.

  • Aqueous transition metal ions are a common example of homogeneous catalysts. They have variable oxidation states and can easily oxidise or reduce other aqueous ions.



Homogeneous Catalysis


Homogeneous is where the catalyst is in the same phase as the reactants. For example, metal ions in a solution where a reaction takes place. As with heterogeneous catalysis, the variable oxidation states of transition metal ions make them very good catalysts.


A common homogeneous catalysis is the reaction between thiosulfate ions and iodide ions, in the presence of Iron(II) ions.


Iodide and thiosulfate ions can react to produce iodine and sulfate ions by the following reaction.

Both the sulfate ions and iodide ions are negatively charged – this makes the reaction very slow as they repel each other. An iron catalyst is used to provide an alternative route.






An Fe  (aq) ion is used, which can be oxidised to form an Fe  (aq) ion. By being oxidised, the ion is reducing another species, in this case the thiosulfate ions.




The thiosulfate ions have been converted into sulfate ions. The Fe  (aq) ions can be reduced to reform Fe  (aq) ions.